In the wireless system, when a mobile terminal that is currently accessing a first node of the wireless network then accesses a second node (different from the first node) of the wireless network, the first node and the mobile terminal deliver to the second node, information related to transmission and reception acknowledgement, which minimizes unnecessary duplicate transmission of data and minimizes transmission delays for new data.
To support broadband wireless access technologies (e.g., WIMAX), there are different types of broadband wireless (air) interfaces, such as cellular 3G technologies (e.g., UMTS, WCDMA, etc.), and multi-carrier based multiple access techniques (e.g., OFDMA, OFDM-TDMA, OFDM-CDMA, etc.). Frequency division multiplexing involves sub-channelization, of which at least four types (OFDM, Flash OFDM, sOFDMA and OFDMA) exist.
Orthogonal Frequency Division Multiplexing (OFDM) involves the splitting of a radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to a receiver. OFDM refers to a form of multi-carrier transmission where all the sub-carriers are orthogonal to each other. Certain IEEE standards and 3GPP standards are related to various aspects of OFDM. For example, portions of the 3GPP standard related to various aspects of Long-Term Evolution (LTE) (such as E-UMTS) are based upon OFDM concepts.
FIGS. 1 and 2 show examples of an Evolved Universal Mobile Telecommunications System (E-UMTS). The E-UMTS system is a system that has evolved from the UMTS system, and its standardization work is currently being performed by the 3GPP standards organization.
As shown in FIGS. 1 and 2, the E-UMTS network is generally comprised of a core network (CN), an E-UTRAN and a terminal (e.g., UE). The E-UTRAN is comprised of a base station (i.e., eNode B or eNB) and an access node (such as an access gateway (AG)) that is located at an end of the E-UMTS network and connects with one or more external networks. The AG can be divided into a portion that handles user traffic and a portion that handles control traffic. In such case, the AG that handles user traffic and the AG that handles control traffic can communicate with each other via a newly defined interface. For one eNode B, one or more cells may exist. Between the eNode Bs, an interface for transmitting user traffic and control traffic may be employed. The core network (CN) may be comprised of nodes and the like that are used for registration and other functions for users of the UEs and the AG. Also, an interface for distinguishing the E-UTRAN and the CN may be employed.
Also, in the E-UMTS network, there may be a control plane server (CPS) that performs radio (wireless) control functions, a radio resource management (RRM) entity that performs radio resource management functions, a mobility management entity (MME) that performs mobility management functions for a mobile terminal. Here, it can be understood that the particular names of the various network entities are not limited to those mentioned above.
The various layers of the radio interface protocol between the mobile terminal and the network may be divided into L1 (Layer 1), L2 (Layer 2), and L3 (Layer 3) based upon the lower three layers of the Open System Interconnection (OSI) standard model that is known the field of communication systems. Among these layers, a physical layer that is part of Layer 1 provides an information transfer service using a physical channel, while a Radio Resource Control (RRC) layer located in Layer 3 performs the function of controlling radio resources between the mobile terminal and the network. To do so, the RRC layer exchanges RRC messages between the mobile terminal and the network. The functions of the RRC layer may be distributed among and performed within the Node B, the CPS/RRM and/or the MME.
The radio interface protocol is horizontally comprised of a physical layer, a data link layer, and a network layer, and vertically comprised of a user plane for transmitting user data and a control plane for transferring control signaling. The radio interface protocol layer may be divided into L1 (Layer 1), L2 (Layer 2), and L3 (Layer 3) based upon the lower three layers of the Open System Interconnection (OSI) standards model that is known the field of communication systems.
FIGS. 3 and 4 show exemplary radio protocol architecture according to the 3GPP radio access network standard. Particular layers of the radio protocol control plane of FIG. 3 and of the radio protocol user plane of FIG. 4 will be described below. The physical layer (i.e., Layer 1) uses a physical channel to provide an information transfer service to a higher layer. The physical layer is connected with a medium access control (MAC) layer located thereabove via a transport channel, and data is transferred between the physical layer and the MAC layer via the transport channel. Also, between respectively different physical layers, namely, between the respective physical layers of the transmitting side (transmitter) and the receiving side (receiver), data is transferred via a physical channel.
The MAC layer of Layer 2 provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 supports the transmission of data with reliability. It should be noted that the RLC layers in FIGS. 3 and 4 are depicted in dotted lines, because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself may not need to exist. The PDCP layer of Layer 2 performs a header compression function that reduces unnecessary control information such that data being transmitted by employing Internet protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio (wireless) interface that has a relatively small bandwidth.
The radio resource control (RRC) layer located at the lowermost portion of Layer 3 is only defined in the control plane, and handles the control of logical channels, transport channels, and physical channels with respect to the configuration, reconfiguration and release of radio bearers (RB). Here, the RB refers to a service that is provided by Layer 2 for data transfer between the mobile terminal and the UTRAN.
As for channels used in downlink transmission for transmitting data from the network to the mobile terminal, there is a broadcast channel (BCH) used for transmitting system information, and a shared channel (SCH) used for transmitting user traffic or control messages. As for channels used in uplink transmission for transmitting data from the mobile terminal to the network, there is a random access channel (RACH) used for transmitting an initial control message, and a shared channel (SCH) used for transmitting user traffic or control messages.
The features of the RLC layer will be considered in more detail hereafter.
A basic function of the RLC layer is to guarantee the quality of service (QoS) for each radio bearer (RB) and to transmit data accordingly. As the RB service is a service provided by the radio protocol of Layer 2 to an upper layer, the entire Layer 2 effects the QoS, but in particular, the effect of the RLC is large. To guarantee the QoS that is unique to the RBs, the RLC has a separate (independent) RLC entity for each RB, and provides two types of RLC modes to support various QoS, namely, unacknowledged mode (UM) and acknowledged mode (AM). These two RLC modes have differences in their operation methods because of the respectively different QoS that is supported, and thus their detailed functions also have differences. Thus, the RLC should be considered according to its operation mode.
The RLC has two modes; a mode (UM) that does not provide a response or acknowledgement with respect to reception of data that was transmitted, and a mode (AM) that provides a response or acknowledgement. The UM RLC transmits each PDU upon attaching thereto a header that includes a sequence number (SN), which allows a receiver to know what PDUs have been lost during transmission. Due to such function, the UM RLC, in the user plane, handles the transmission of broadcast/multicast data or the transmission of real-time packet data such as voice (e.g., VoIP) and streaming of the packet service (PS) domain, while in the control plane, handles the transmission of RRC messages that do not need reception acknowledgement among the RRC messages transmitted to a particular terminal within a cell or a particular terminal group.
Similar to the UM RLC, the AM RLC forms a PDU by adding a PDU header that contains a sequence number (SN), but unlike the UM RLC, there is a large difference in that the receiver provides an acknowledgement for a PDU transmitted from a transmitter. In AM RLC, the reason why the receiver provides acknowledgement is to allow the receiver to request re-transmission of those PDUs that it had not properly received, and this re-transmission function is the most significant characteristic of the AM RLC. As a result, the purpose of the AM RLC is to guarantee error-free data transmissions through use of re-transmissions, and due to this purpose, the AM RLC typically handles non-real-time packet data transmissions, such as TCP/IP of the PS domain, in the user plane, and handles the transmission of RRC messages that need reception acknowledgement among the RRC messages transmitted to a particular terminal within a cell.
With respect to directional aspects, the UM RLC is used in uni-directional communications, while the AM RLC is used in bi-directional communications because there is feedback from the receiver. Because such bi-directional communications are typically used in point-to-point communications, the AM RLC only employs a dedicated logical channel. There are also differences in structural aspects, whereby the UM RLC has a single type of structure comprised of one RLC entity either for transmission or for reception, but for the AM RLC, both a transmitter and a receiver exist in a single RLC entity.
The reason why the AM RLC is complicated is due to its re-transmission function. For managing re-transmissions, the AM RLC has a re-transmission buffer in addition to a transmission/reception buffer, uses a transmission/reception window for flow control, uses a polling function whereby the transmitter requests a status information from the peer RLC entity in the receiver, uses a status report for the receiver to report its buffer status to the peer RLC of the transmitter, uses a status PDU for carrying status information, uses a piggyback function that inserts a status PDU into a data PDU to increase data transmission efficiency, and performs various other functions. In addition, if the AM RLC entity finds a serious error in its operation procedure, a Reset PDU for requesting the peer AM RLC entity to re-set all operations and parameters, and a Reset Ack PDU for responding to such Reset PDU are employed. Also, to support these functions, the AM RLC requires various types of protocol parameters, state variables, and timers. The PDUs used controlling data transmissions from the AM RLC, such as the status report or status PDU, Reset PDU, and the like, are called Control PDUs, and the PDUs used for delivering user data are called Data PDUs.
A mobile communications system is different than a wired network in that services must be seamlessly provided to terminals that have mobility. Namely, situations where the terminal moves from one region to another different region should be supported. If the terminal moves away from a base station that it is currently accessing and moves closer towards to a new base station, the network should perform the operations to move the access point of the terminal to the new base station. While the terminal disconnects its access with the previous base station and until the access to a new base station is completed, data transmission or reception cannot be performed.
However, all user data has a time limit. For example, for a voice call, voice data (or information) must be transmitted to the recipient within a set time period. Also, data such as TCP must be sent from a transmitter and received by a receiver within a certain time period, and the receiver should send a reception acknowledgement to the transmitter within a certain time period. Otherwise, the transmitter will re-transmit the TCP data.